1. Field of the Invention
The invention relates to a battery assembly composed of multiple electrochemical cells networked in a configuration suitable for powering electric vehicles (EVs) or hybrid electric vehicles (HEVs). This configuration not only improves the reliability of the power source, but also increases the power and energy densities of the battery assembly.
2. Related Art
Battery-powered vehicles such as electric vehicles (EVs) or hybrid electric vehicles (HEVs) contain one or more batteries that are constructed from multiple electrochemical cells. For example, a conventional lead-acid battery made for vehicle applications contains six cells connected in series inside a housing to form a twelve-volt power source. In most cases, neither the voltage nor the energy content of a single battery will be sufficient to power passenger electric vehicles; therefore, a solution is to connect a quantity of such batteries in series to boost both the voltage and energy content. In essence, this battery assembly contains a single string of N cells connected in series. When each cell has a voltage of V volts and a capacity of C ampere-hours, the total energy of the assembly is calculated to be N×V×C (N times V times C) watt-hours.
Although simple in concept, this design suffers from poor reliability and flexibility. For example, a practical battery assembly of 300 volts and 25,000 watt-hours is made by connecting in series twenty-five lead-acid batteries, each containing 1,000 watt-hours of energy. Since each of the twelve-volt batteries contains six cells in series, this assembly has one hundred fifty cells connected in a single string. If any of the cells or cell connections fails, the whole assembly fails, which presents a significant reliability challenge. Additionally, lead-acid batteries are very bulky. Electric vehicles usually have limited space set aside for their battery assemblies. The volume of a 1,000 watt-hour battery is nearly ten liters. Sparing two hundred fifty liters of volume for the 25,000 watt-hours necessary to run an electric vehicle's power source will not leave much flexibility for EV design.
On top of the above-mentioned issues, lead-acid batteries are inordinately heavy. In order to achieve a desired driving range, a battery assembly as heavy as half of the vehicle weight is often required. For example, in the scenario presented above, the weight of the battery assembly would be 750 kilograms excluding connectors and housing.
In recent years, new cell chemistries with better specific energy densities than lead-acid chemistry have emerged. For example, nickel metal hydride cells can deliver 50 to 70 watt-hours/kilogram, a significant improvement over 30 to 40 watt-hours/kilogram that lead-acid cells can deliver. Lithium-ion cells can deliver 100 watt-hours/kilogram or higher. These new chemistries have already been successfully commercialized in low-energy and low-power applications such as notebook computers and mobile phones. But, EV or HEV applications where high-power and high-energy are required are still in experimental stages. In addition to cost and safety concerns, the lack of a battery design that can efficiently harness the cell's power and energy is the primary reason for the slow commercialization.
In essence, a battery assembly can be structured by connecting cells in series or in parallel or a combination of both. The larger the amount of energy contained in each individual cell, the fewer the cells needed to construct the battery. The fewer the cells needed to construct the battery, the fewer parts needed for connecting the cells which will serve to reduce the weight of the finished assembly. Therefore, the larger the amount of energy contained in each individual cell, the lower the weight of the final assembly. The weight reduction is particularly obvious when high power output is required; heavy connectors must be used in order to minimize the power loss due to added resistance introduced by them. For EV applications, a battery power output of 150,000 w or higher is not unusual. For comparison purposes, a typical notebook computer battery has a maximum power of less than 200 w. Even at this low level of power, the notebook computer battery's assembly can overheat if not properly designed. Hence careful engineering of connectors in EV battery is crucial to its success.
As the power and energy content of a cell increase, safety concerns increase as well. In extreme situations, a battery can cause a fire or explosions that may spread throughout the entire pack to create a major safety incident. Such is the case for lithium-ion cells. Although they have the advantages of high specific energy and power densities, the application to EV or HEV still lags behind nickel metal hydride cells due to safety matters.
It is well known that safety concerns for lithium-ion cells become less of a concern when the size of the cells is reduced. But, the conventional thinking argues that building large battery assemblies from small cells would inevitably be ineffective due to a large number of connector required, which will result in both reduced specific energy and reliability. With reference to FIG. 11, a generic battery connects P cell strings (62) in parallel between two current collectors (63)(64) where each cell string (62) includes S cells (61) connected in series. It is desirable for the current collectors (63)(64), which carry the battery current to external loads, to be large conductors in order to be less resistive. However, the connectors (65) for the series string do not need to be as large as current collectors (63)(64) because on average the current flow in individual cell strings is 1/P (one over P) of that flowing in the current collectors (63)(64). As a result, this assembly is shown to be efficient in terms of weight added by the connectors (65), particularly when cells (61) are small and S is a large quantity. However the reliability is an issue. As explained earlier, when one cell (61) fails, the entire string (62) fails, which reduces the energy by a factor by 1/P. The power density suffers because of this as well. When a second cell (61) in the assembly fails, the probability is higher for it to be located on a different cell string (62) than on the same string. The energy of the assembly would then be further reduced by a factor of 2/P.
When this design is applied to lithium-ion cells, the battery charge/discharge management system is complex. Lithium-ion cells perform the best when the charge and discharge are controlled within a voltage range. If the range is exceeded during use, the cells can suffer reduced life and capacity, or even become a safety hazard. In this assembly design described, S×P (S times P) cells need to be controlled individually, which would be a cumbersome task for large assemblies.
To simplify the battery management system, an alternative design as shown in FIGS. 12 and 13 connects P cells (71) in parallel with two current collectors (73) and then connects S parallel groups (72) in series. All P cells (71) connected within a parallel group (72) will be made to display the same voltages by their current collectors (73) and as a result, the management circuitry will need to control only S points as opposed to S×P points as in the first design. Unfortunately, with this design, each parallel group (72) will be required to gather its own current by heavy current collectors (73), which will make the assembly heavy.
Clearly, a design suitable for EV or HEV batteries that possesses a combination of high energy, high power, high reliability and high level of safety is lacking.